Originally Posted by Orlgin

Haste is great for Duel-Wielding Death Knights who use spells as their primary source of damage. You don't benefit from haste as much because you use a two-handed weapon and use weapon strikes that don't get their GCD reduced by haste. For example, a 0/20/51 values haste very highly because with enough haste, they can use 7 GCDs in 10 seconds in Blood Presence. The pets (ghoul and gargoyle) both benefit from the haste as well. The set was designed with stats that would benefit all builds. The haste benefits duel-wield but the set bonuses favor two-handed. Balanced.

When examining the stat values for any build, it is important to notice these things. The values for even a 51/13/7 are different from a 51/0/20. Some things you can take for granted. Strength, for example, is the best stat for all builds after hit capping for spells. Once you know your specific values, you can then judge your equipment accordingly.